This application claims the benefit of priority from French Application No. 0308865, filed Jul. 21, 2003, which is incorporated herein by reference.
The general field of the invention is that of wafer level fabrication of microstructures, for example by means of micromachining or chemical processing techniques used in microelectronics (deposition and etching of layers, photolithography and so on). The invention relates more particularly to certain microstructures of the micro-electro-mechanical system (MEMS) type, such as various sensors and actuators, which are obtained by freeing mobile portions (for example membranes or seismic masses).
To obtain such microstructures, the starting material may be of the silicon-on-insulator (SOI) type, for example, which usually comprises a surface layer of silicon and an underlying buried layer of silicon oxide SiO2.
There are several ways to fabricate the SOI material. See, for example, “Semiconductor Wafer Bonding”, Q. Y. Tong and U. Goesele, Science and Technology, ECS Series, John Wiley, New Jersey 1999. However, most SOI materials are nowadays fabricated by the molecular bonding technique. For example, two silicon plates are bonded together by the molecular bonding technique, at least one of the two plates having a surface layer of silicon oxide. The silicon oxide layer is usually produced by thermal oxidation. One of the two plates is then thinned. An SOI type structure is obtained in this way.
Several techniques for obtaining a thin layer may be used (in the context of the present invention, a layer is regarded as thin if its thickness is less than a few tens of microns). For example, a first technique is thinning (mechanical thinning by planing and/or smoothing, and/or chemical thinning, and/or mechanical-chemical thinning). A second technique uses fracture in a fragile area created at a certain depth in one of the two plates, prior to said molecular bonding, for example by implanting one or more gaseous species; the patent application FR-2 681 472 discloses a method of the above kind, which at present is known as the “Smart-Cut®” method (see, for example “The Generic Nature of the Smart-Cut® Process for Thin-film Transfer”, B. Aspar et al., Journal of Electronic Materials, Vol. 30, No 7, 2001). These methods are very suitable for obtaining thin surface layers of silicon, usually less than 2 μm thick.
It is possible to produce mobile or deformable mechanical structures from this SOI material, for example by machining the top silicon film and freeing the structure by chemically etching the whole or a portion of the underlying oxide; for example, the mechanical structure is created by plasma etching the thin surface layer of silicon and chemically etching the silicon oxide layer using hydrofluoric acid (HF).
In the context of the present invention, a layer forming part of a stacked structure is referred to as a sacrificial layer when it can be eliminated subsequently, for example during use of the stacked structure to fabricate a component having a mobile or deformable portion. The material constituting a sacrificial layer is therefore different, from the chemical or crystallographic point of view, from the material constituting the non-sacrificial layers, i.e. the layers intended to remain after eliminating the sacrificial layer. For example, if the stacked structure is made from an SOI material, the silicon oxide layer serves as a sacrificial layer and the silicon layers serve as non-sacrificial layers.
This process is relatively simple to use and produces a variety of microstructures.
Pressure sensors of high quality may be produced in this way, for example.
The accelerometer disclosed in the patent FR 2 558 263 may be cited as another example of this kind of microstructure and comprises, within a thin layer, a first portion cut out from the thin layer and a second portion consisting of the remainder of the thin layer, the first portion being connected to the second by means of flexible beams allowing the first or sensitive portion to move with a certain amplitude in the plane of the thin layer. This device is used to measure acceleration of any system to which it is attached by means of a variation in electrical capacitance caused by said movement.
Other detailed examples of such microstructures can be found in “SOI ‘SIMOX’; from bulk to surface micromachining, a new age for silicon sensors and actuators”, B. Diem et al., Sensors and Actuators, Vol. A 46-47, pages 8 to 16 (1995).
However, fabrication of such microstructures runs up against the following problem. During the production of the structure, and in particular at the time of drying the rinsing liquid after chemical etching with hydrofluoric acid, capillary forces between the surfaces and the liquid are very high and lead to partial or total sticking of the freed structures; another cause of sticking is a solid deposit which can be produced by said drying. In the case of the accelerometer described above, for example, this leads to the beams sticking to the substrate constituting the bottom of the cavity containing the device, which obviously prevents the beams from moving as intended in response to acceleration of the system.
The SOI structure fabrication techniques referred to above lead to interfaces between the surface layer of silicon and the buried oxide, and between the buried oxide and the substrate that are not particularly rough. This sticking problem is aggravated in that nowadays SOI structures are produced with very smooth interfaces; the thinner the oxide film, and the larger the structures to be freed, the greater the problem.
In order to avoid these problems of unwanted sticking, it is necessary to take important precautions, which make the freeing process complex, costly and difficult to control. Moreover, for reasons of reliability, such unwanted sticking of facing faces within MEMS components after the components go into service has to be prevented.
A first prior art means of preventing such sticking consists in reducing the bonding energy of the freed layer and the substrate. However, these techniques employ methods of chemical preparation of the surfaces that are incompatible with the high temperatures usually required for subsequent MEMS fabrication steps. For more details, see “Suppression of Stiction in MEMS”, C. H. Mastrangelo, Proceedings of the Materials Research Society Seminar, Vol. 605, 2000.
A second prior art way to prevent this sticking is to make the effective area of contact small when these two surfaces move toward each other. A method of this kind is disclosed in the patent FR 9 508 882. It consists in holding the freed layer and the substrate at a distance by etching the intermediate sacrificial layer to create abutments on each of the facing faces of the freed layer and the substrate.
Another such method is described in “Surface Roughness Modification of Interfacial Contacts in Polysilicon Microstructures”, R. L. Alley et al., Proceedings of the 7th International Conference on Semiconductor Detectors and Actuators). That paper proposes a method of producing partially mobile components including steps leading to a component whose facing free faces have a roughness adapted to prevent unwanted sticking between said faces (see the paper for a statistical definition of roughness; for example, roughness may be measured using an atomic force microscope scanning areas 1 μm×1 μm, for example). During the step of chemical freeing of the structure, this method roughens the surfaces concerned in order for the effective area of contact to be limited to the summits of the asperities of those surfaces. The paper by R. L. Alley et al. is essentially concerned with assessing how the sticking force decreases when the roughness increases.
The method described in the above paper has the drawback that it cannot be used to produce certain types of components. In particular, the method provides for the deposition of a surface film on the substrate of the stacked structure; the person skilled in the art knows that this deposition is not always possible, for example depending on the materials concerned. For example, this method cannot produce a monocrystalline surface film to be freed if the material of the sacrificial layer is amorphous; nor can it produce a monocrystalline film, for example of silicon, on a sacrificial layer of a polymer material, because of the incompatibility of the temperature for depositing the silicon film and the temperatures that a polymer is usually able to withstand.